1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly relates to a semiconductor device using three-dimensionally formed transistors or thyristors and a manufacturing method of the semiconductor device.
2. Description of Related Art
Improvement in an integration degree of semiconductor devices has so far been primarily achieved by downsizing of transistors. However, the downsizing of the transistors has become almost to the maximum possible extent, and any further downsizing is likely to hamper correct operations of the semiconductor devices due to the short channel effect or the like.
As a fundamental solution to such a problem, a method has been proposed by which a semiconductor substrate is subjected to three-dimensional processing to form the transistor three-dimensionally. Particularly, a type of three-dimensional transistor in which a silicon pillar that extends perpendicular to a principal surface of the semiconductor substrate is used as a channel, has advantages of having a smaller occupied space and a larger drain current due to complete depletion. This type of transistor can also realize a close-packed layout of 4 F2 (F is the minimum processing dimension) (see Japanese Patent Application Laid-open No. 2009-010366).
Among these semiconductor devices, particularly in a DRAM (Dynamic Random Access Memory), capacitors are typically used for storing information. However, with the improvement in the integration degree, downsizing of the capacitor has also become imperative, resulting in difficulty in securing an adequate capacitance from the capacitor. Therefore, in recent years, there have been proposed capacitorless DRAMs that store information in thyristors or transistors instead of capacitors. An example of such a capacitorless DRAM that stores information in three-dimensionally formed thyristors is disclosed in United States Patent Publication No. 2009/0213648.
Meanwhile, in the semiconductor devices that use three-dimensionally formed thyristors or transistors for storing information, an amount of electric charge accumulated therein is smaller than that of conventional devices due to downsizing. Therefore, the influence of a leakage current on information retention characteristics increases, and it becomes necessary to reduce the leakage current of the thyristors or transistors.